An ink jet head, which is a liquid ejection head, is configured to inject liquid droplets by changing an ink pressure in a pressure chamber to cause ink to flow so that the ink is ejected from an ejection orifice. In particular, a drop-on-demand type head has been most widely used. Further, a system for applying a pressure to ink is roughly classified into two systems. One system involves changing a pressure of ink by changing a pressure in a pressure chamber with a driving signal to a piezoelectric element, and the other system involves applying a pressure to ink by generating air bubbles in a pressure chamber with a driving signal to a resistor.
An ink jet head using a piezoelectric element can be relatively easily produced by machining a bulk piezoelectric material. Further, the ink jet head using a piezoelectric element has another advantage in that there is relatively little restriction on ink, and a wide range of ink materials can be applied selectively onto a recording medium. From the foregoing point of view, in recent years, there is an increasing attempt to use an ink jet head for industrial purposes such as the production of a color filter and the formation of wiring.
In a piezoelectric ink jet head for industrial use, a shear mode system has often been adopted. The shear mode system involves applying an electric field to a polarized piezoelectric material in an orthogonal direction to subject the piezoelectric material to shearing deformation. A piezoelectric portion to be deformed is a partition wall portion formed by processing a polarized bulk piezoelectric material with a dicing blade so as to form an ink groove or the like. Electrodes for driving a piezoelectric element are formed on both sides of the partition wall, and a nozzle plate having a nozzle formed therein and an ink supply system are formed, with the result that an ink jet head is formed.
As a shear mode type ink jet head, there is an ink jet head formed of an ink groove containing ink and an air groove not containing ink adjacent to the ink groove, as described in Patent Literature 1. A partition wall between the ink groove and the air groove is deformed by grounding the electrode on the ink groove side and applying a signal voltage to the electrode on the air groove side. The ink groove, which is in contact with ink, is grounded in this system, and hence ink having high conductivity can be used (see Patent Literature 1).
In recent years, there is a demand for high definition patterning in a liquid ejection device. Therefore, it is necessary that ejection liquid droplets be miniaturized. The amount of liquid droplets to be required is about sub pL to several pL. In general, the size of a liquid droplet is about the size of a nozzle diameter. Then, in order to form a liquid droplet smaller than a nozzle diameter, there has been considered a method using meniscus driving of controlling meniscus at high speed. For example, Patent Literature 2 describes a method of controlling meniscus so as to form a liquid droplet of 1 pL or less with respect to a nozzle diameter of φ20 μm or less. Specifically, Patent Literature 2 defines a voltage change amount and a voltage change time in a voltage change process so as to control a drawn-in amount of meniscus.
As described in Non Patent Literature 1 regarding parameters of an ejection amount and a liquid ejection head in a shear mode type liquid ejection device, according to the simplest driving (push-ejection) method for ejection using the resonance of a liquid chamber, the ejection amount becomes as follows: Ejection amount=π×(nozzle diameter)^2×(liquid droplet velocity)/2/Fr (resonance frequency of a liquid chamber). Further, when a driving (pull-ejection) method for miniaturizing a liquid droplet is performed, the ejection amount becomes as follows: Ejection amount=π×(nozzle diameter) ^2×(liquid droplet velocity)/4/Fr (resonance frequency of a liquid chamber). Thus, the amount of liquid droplets can be reduced to about a half. Further, the ejection amount can be reduced to about 30% by controlling the application of a pulse in the above-mentioned driving waveform. Thus, the ejection amount can be reduced to about several pL and controlled stably to some degree by the driving method.
However, it is very difficult to stably eject liquid droplets of about sub pL to 2 pL with a nozzle diameter of about φ20 μm by a driving method in a liquid ejection device using piezoelectric driving. For example, as described in Patent Literature 3, when the velocity of main liquid droplets is set to a certain velocity or more, minute liquid droplets are separated at high speed before ejection of the main liquid droplets, depending on a driving waveform, and thus it is difficult to control the ejection amount.